Automotive vehicle steering systems are subject to a great number of forces that may cause noise, vibration and harshness (NVH) heard and felt by the vehicle driver and passengers. Steering NVH is particularly a problem when the vehicle is driving over rough road surfaces, such as cobblestones or Belgian blocks.
Previous efforts to reduce or eliminate steering NVH over rough road surfaces focused on designing and tuning the bushings, bearings and joints used through the steering and suspension systems to achieve a desired level of NVH. Unfortunately, these efforts have had only limited success, and they often require compromise in other vehicle attributes, such as ride, steering feel, rolling comfort and impact harshness.
The problems with steering mechanism NVH encountered when a vehicle is driven over a rough surface are shared by both rack and pinion and worm and wheel steering mechanisms. Being that the rack and pinion steering mechanism is the more common of the two, more research has been directed to solving the problems of this type of mechanism.
For example, DE 10162470A1 is directed to a steering gear for a motor vehicle in which a thrust piece by means of which the toothed rack is pressed against the drive pinion is guided in a longitudinally movable manner within a connecting piece of the steering gear housing. Accordingly, the preload force acts on the thrust piece in such a way that the direction of action of the preloaded force acting on the thrust piece and the movement direction of the thrust piece enclose an angle. The objective of this reference is to prevent rattling noises as a result of alternating contact with the thrust piece.
By way of further example, U.S. Pat. No. 7,654,166 discloses a toothed-rack steering gear having a component which serves to automatically compensate play which exists between surfaces which face toward one another of a spring-loaded thrust piece of a counter bearing against which the spring acting on the thrust piece is supported. According to the invention disclosed in this patent, a two spaced apart, wedge-shaped intermediate components are arranged between the thrust piece and the counter bearing. The wedge-shaped intermediate components are attached to one another by an elastic element. The wedge shape and the force with which the components are attached to one another by the elastic element compensate for play that exists between the thrust piece and the counter bearing.
While these innovations represent improvements in the art of steering mechanisms, a particular challenge faced by steering mechanism designers is that increasing preload between moving components results in reduced NVH but results in other difficulties. Particularly, the amount of preload and clearance in a steering mechanism must be set to provide low friction for good steering without causing mechanical noises during steering inputs or during road inputs. A lower clearance or higher preload results in less noise but results in an undesirable increase in friction.
Complicating engineering development is the fact that the reduction of NVH is not necessary under all conditions. Particularly, under ordinary driving conditions, the increased preload is not necessary. Thus a practical steering mechanism that is selectively adjustable for clearance and preload depending on surface conditions is desirable.
Accordingly, as in so many areas of vehicle technology there is room for improvement related to steering mechanisms in automobiles.